Assuming your DiskStation supports transcoding (not all do), all you need are the FLAC files. The DS Audio app (at least on iOS) will let you choose to have the files transcoded to WAV or MP3 (as will Audio Station when the remote computer/browser doesn't support FLAC). I am speaking theoretically here, since I don't have any FLAC material.
DS 1512+ w/3GB, 5 x 3TB Seagate ST3000DM001 8.2TB RAID 6, half files/half Time Machine.
Icy Dock MB559U3S-1SB enclosure w/4TB Hitachi UltraStar via USB 3 for files backup
UVERSE to AirPort Extreme + 2 AirPort Express w/speakers. TRENDnet TV-IP312WN camera
CyberPower CP1500PFCLCD Sine Wave UPS
DSM 4.1-2661 w/SSH + SFTP, VPN Server, Syslog Server, Media Server, Mail Server, Mail Station,
Audio Station, Surveillance Station, Photo Station, Web Station - DS Apps on iPad & iPod Touch.